Building a Garden Office Advice Required

[Markymark]

Hi

I have a problem. I have got to the point where I am fitting 2.5mm Twin & Earth Cable for the ring main. I Have fitted it straight down the middle well away from the studs just in case it gets hit by any fixings. I am reading all over that it is a risk to install electrical wiring within thermal insulation and I would agree. The build has enough combustible surfaces including being under a canopy of Pine trees 60 ft high to cause a lot of problems if it went up in flames. I read there is a BRE publication that explains all I need to know out working with thermal insulation and electrics but it is £39.00. Why oh why do they charge for safety information????

Can anyone help me as I have filled in between all the other studs with 90mm Xtratherm and I am about to install insulation between the studs with the cables running through?


Thanks Mark

 

[Cegidfa]

Hi Mark,
Have a read of the info below for the latest view.
If the wiring is in a wall, the heat 'duty' is less onerous than in the roof, which will have a greater amount of insulation.
As the guide says, you could use a higher temp. rated cable, or, up the size a notch, as stated.

Regards...Dick.

http://WWW.electrical.theiet.org/wiring-matters/15/current-ratings.cfm?type=pdf

 

[imageel]

Mark - I think there are derating tables in the IEE 17th edition reference thingy I have - will have a look and post what the formula is. Are you sure that you are putting the cable runs in acceptable places? - there are regs for the zones where they are permitted to run (roughly in line vertically or horizontally from your sockets/switches.
One question though - why are you insulating this void - surely 90mm insulation in your neck of the woods is sufficient, and putting another layer may cause condensation problems since your cable exits to your sockets will pierce your vapour barrier -won't it?

 

[Markymark]

imageel-

Thanks for the reply. I have a electrician come round as he is certifying it under Part P. He wanted to see the wires in the structure before the insulation or plasterboard. He has looked and said he is happy. I should have asked the question with him before he disappeared. He is coming back to install the armour cable between the garage and the shed, I am trying to get things moving so feel a little impatient.

With regards to the insulation. This is the only insulation in the wall. The studs are 90 x 42 PAR ans so it made sense to fill in between the studs with solid insulation. I agree with the sockets effecting the vapour barrier but I am not sure how I would get round this? I can only assume putting expensive plastic surface trunking around the walls but that would look unsightly and would be expensive.

Dick - Thanks for the info I will take a look. I have standard 2.5mm T&E cable installed and some people say that the expanded foam can react with the PVC sheathing on the Cable. Also any heat created by current flow will only collect in the cable and possibly overheat? Does 200mm of rockwool have a higher value than 90mm Celotex or equivalent foil backed expanded foam insulation?


I'll look forward to your post.

Mark

 

[imageel]

Mark,
Perhaps I have misunderstood - so from the outside wall inwards you have -
cladding, permeable membrane, studding (infilled with insulation), vapour barrier,small void for cabling, whatever material to finish presumably ply/plasterboard.
I thought you were thinking of filling your service void i.e. around the cables, with insulation, apologies if I was mistaken!

 

[Cegidfa]

Hi Mark,
The answer is that 200mm of rockwool is .21w/mk and 90mm of PIR is .24w/mk (approx). So the rockwool wins.
I too have read of the reaction between foam and cable. If you have some 50mm tape left why not cover the cables in it.
Or, staple some polythene/plastic over the cables, as an interface, and then add the foam?
As to heat 'collecting' in the cable, the cables are presumably clipped to the studding? Don't lose sight of the fact that wood is a 'thermal bridge'
these days. and the insulation is on one side only And you won't be running high current machinery all day/every day...will you :shock:

I have had the loft cables under 500mm....yes, 500mm of rockwool for some time now, and the loft is as hot as Hades...naff all ventilation of any worth.
I have monitered the effect, i.e. discolouring/ stiffening,etc and found none to date. Yes, if you seriously overload/overheat cable consistently, you are asking for trouble,
but in a domestic situation such as yours, you are unlikely to have a problem. I had an unconventional approach to what was and wasn't 'safe' at work.
It's all about undestanding your environment, and using that knowledge wisely, not following 'rules' blindly.

Regards...Dick.
PS. With regard to piercing the vapour barrier when fitting the sockets.....don't worry, unless you are going to have a bath in there :shock:
It is not a house....I'm the anal one here, and I won't be fitting a vapour barrier, it's a waste of money.

 

[StarGazer]

The reaction between insulation and the PVC buffer on mains cable is for expanded polystyrene (white crumbly stuff) which leaches the plasticiser out from the grey/white outer insulation on mains cable.

Mains cables are not affected in the same way by PIR foam (celotex etc) insulation boards.

If the cables are buried in the insulation in the walls then they should be derated as another poster has already stated.

 

[Markymark]

Mark,
Perhaps I have misunderstood - so from the outside wall inwards you have -
cladding, permeable membrane, studding (infilled with insulation), vapour barrier,small void for cabling, whatever material to finish presumably ply/plasterboard.
I thought you were thinking of filling your service void i.e. around the cables, with insulation, apologies if I was mistaken!

imageel - From the outside in Shiplap, 11mm OSB 3, Studs, Rigid insulation in between Studs, Vapour Barrier, OSB, 9mm Bead & Butt Board.

The mains cables run down the between the Studs. Not attached to the studs. through to a metal plate supporting the steel back boxes.

I will get a photo on soon.

To fit the Insulation I would have to cut a piece in half and trap the two cables in between it like a sandwich. Alternately I can cut a channel out of the insulation for the cables to sit in and loosed some of the value of the insulation.

Hope that is a little clearer? It should be clear on the photo appearing.

Thanks all

Mark

 

[Markymark]

Thanks Star Gazer

That seems plausible. I think the total current capacity to the office is derated as the feed to the garage is too small so the sparky is puting the garden office on a 16amp MCB.

Mark

 

[Markymark]

This is a photo of the corner with the wiring running down the centre between the studs.

 

[Markymark]

Thanks everyone so far.
Everyone's help has got me this far.


I have finished the insulation and vapour barrier which I Took behind the socket backs. I am cladding the walls, due to the fact that my studs are at 2ft centres to allow easy fitting of 8 x 4 sheeting on the outside, internally with yet more 11mm OSB. Then to avoid the plasterer, I will install 9mm butt & Bead MDF panels. This should give it a T& G appearance. I was going to install the OSB on the ceiling also but wondered if I could have some ideas from anyone on how to finish the ceiling off. Don't fancy the Bead & Butt panels on the roof also so not sure how to finish the ceiling???


Any ideas?

Mark

 

[bosshogg]

Mark why not fit plasterboard , OK at 2' centres you'll have a bit of cutting, mind you only 1:1/2" or so if you use the 2.4 x 1.2 boards. You'll need two to fix these to the ceiling joists/collars, and dwangs/noggins will have to be fitted in straight lines at 1.2 centres, it's something of a learned technique, but the result is a normal looking ceiling...bosshogg

Ethical axioms are found and tested not very differently from the axioms of science. Truth is what stands the test of experience. A.E. (hammer)

 

[Markymark]

Thanks Boss

Good Idea, but I am trying to keep trades down to a minimum due to costs escalating. I was going to try a panelled ceiling. I was going to arrange the boards symmetrically and use a small timber trim to cover the joint and so give it a panelled look. What do you think?

Mark

 

[bosshogg]

Sticking with plasterboard, which in my opinion is the way to go, you can buy small sheets which a single pair of hands can install and to assist you can make up a pair of soldiers ('T' topped sticks) which you position as you progress, pull under the sheets, to jamb them in place whilst you screw them up. I could talk you through hand taping the joints if you want, or you can use square edge boards covering the joints with thin straps too form a Jacobean type of ceiling. Every bit as easy as what you are already considering...bosshogg

I never teach my pupils. I only attempt to provide the conditions in which they can learn. A.E. (hammer)

 

[belcher]

i come at things from a fencing background so my approach may be way off but all i did was concrete six large posts(5x4)3m long in the ground about 800mm then build your shed off of those,no need to dig out soil to level the ground,bolt on your floor supports, bolt on your roof using a single slope roof,sometimes called a pent shed design. Nail your shiplap to the outside,nail your ply to the inside,cut off what you dont want from the top of the posts.

 

[MickCheese]

For the ceiling I would go with plasterboard. 8X4 sheets are not a one man job but with some made supports you can do it with two. Then just tape and plaster the joints, a heavy lining paper will give a good surface to paint.

Just need a friend for a day to help.

Good luck.

Mick

 

[Dibs-h]

8X4 sheets are not a one man job

I can testify to that! :mrgreen:

Dibs

 

[No skills]

8X4 sheets are not a one man job

I can testify to that! :mrgreen:

Dibs

I wish somebody would convince my boss of that, who thinks we should be able to fit 8x4 12mm ply up to celings single handed (and quickly!) :lol:

 

[bosshogg]

8X4 sheets are not a one man job

I can testify to that! :mrgreen:

Dibs

I wish somebody would convince my boss of that, who thinks we should be able to fit 8x4 12mm ply up to celings single handed (and quickly!) :lol:

H&S say no and it's your right to contact them to have this practice stopped. That applies to anyone working in the construction industry, a risk assessment would identify this and show another method would HAVE to be found...bosshogg

Just as a foot note, anyone knowingly performing illegal duties is doing so against the law (Regs in a court of law - are laws) (hammer)

 

[Cegidfa]

Well said Boss,
The number of times that I was told to bypass a door safety switch...or else, by managers and Team Leaders, are inumerable.
Funny how the threat vanished when they were asked to 'put it in writing" on official company paper.
And this on lethal equipment, such as a massive filling machine canning at 1500 cans per minute.

Regards...Dick.